Masonry Magazine April 1982 Page. 21
CORBELS AND RACKING
# General
Corbeling of brick masonry may be done to achieve the desired esthetics, or to provide structural support. There are empirical requirements provided by most codes and standards for unreinforced corbels, as shown in Fig. 8. If these requirements are to be exceeded, then the element will require a rational design as a reinforced element.
Corbels. The empirical approach requires that the total horizontal projection not exceed one-half the thickness of a solid wall, or one-half the thickness of the veneer of a veneered wall. It is also required that the projection of a single course not exceed one-half of the unit height or one-third of the unit bed depth, whichever is less. From these limitations, the minimum slope of the corbeling can be established (angle measured from the horizontal to the face of the corbeled surface is 63 deg 26 min, see Fig. 8). The required slope could be increased by the requirements that the unit projection not exceed one-third of the bed depth if they are more restrictive. It should be pointed out that the eccentricity induced into the wall by the corbeling must be considered in the wall design. If these limitations are exceeded, the wall should be reinforced to resist the stresses developed by the corbeling.
Fig. 9 illustrates graphically the pattern of stresses within two corbels of different configurations under identical loading conditions. The corbel on the left is 45 deg from horizontal, which is not in accordance with building code requirements. The corbeled wall on the right has an angle of corbel 60 deg from horizontal and is very close to the building code requirement of 63 deg 26 min discussed above. The 60-deg corbel shows a stress pattern with axial and shear stresses with the only concentration of stresses directly below the applied load, P. The shear stresses are well distributed within the wall section. The 45-deg corbel, on the other hand, has bending stresses in addition to the axial and shear stresses, and the pattern of the stresses has been drastically altered. In addition to the concentration of compressive stresses immediately beneath the load, P, there is another concentration of compressive stress at the toe of the corbel. The bending stresses require that a corbel of this configuration be rationally designed and reinforced. Those corbels having an angle from horizontal of 60 deg or greater do not require reinforcement unless they exceed the other requirements given above.
Racking. When racking back to achieve the desired dimensions, care must be exercised to insure that, since there is no limitation on the distance each unit may be racked, the cores of the units are not exposed. Preferred construction consists of a setting bed over the racked face with the uncored brick or paving brick set to provide a weather-resistant surface. Mortar washes may also be used. They may not, however, be as durable. When using a mortar wash, it should not bridge over the rack, but should fill each step individually.